CVE-2026-28975
CVE-2026-28975 is a security vulnerability in github.com/apple/swift-nio-extras. O3 Security confirms whether CVE-2026-28975 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
NIOExtras: NIOHTTPRequestDecompressor ratio limit bypass via inflated Content-Length
Real-World Exposure
github.com/apple/swift-nio-extrasReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects SwiftURL packages — download data is not available via public APIs for these ecosystems.
Description
Impact
When NIOHTTPRequestDecompressor is configured with .ratio(N), the decompression limit is enforced using the Content-Length header value from the incoming request rather than the actual number of compressed bytes received. Since Content-Length is attacker-controlled, a malicious client can supply an inflated value that causes the ratio check to always pass, effectively disabling the configured decompression limit.
This allows an attacker to send a small, highly-compressed payload (a "gzip bomb") with a falsified Content-Length header to bypass the ratio-based protection entirely. The server will decompress the payload without limit, consuming unbounded memory and potentially causing denial of service.
For example, a gzip payload containing highly repetitive data can achieve amplification ratios of several hundred to one. Under .ratio(10) such a payload should be rejected, but if the attacker sets Content-Length to match the decompressed size, the check evaluates decompressed > decompressed * 10 which is always false, and the payload is accepted without error.
Across repeated requests, this allows sustained memory amplification far exceeding the configured limits with no error raised.
Relationship to CVE-2020-9840
GHSA-xhhr-p2r9-jmm7 (CVE-2020-9840) found that the .size limit checked compressed rather than decompressed bytes and recommended .ratio as a workaround. This advisory identifies a distinct flaw in the .ratio limit itself: it uses the attacker-supplied Content-Length header as the denominator rather than actual consumed compressed bytes. The two vulnerabilities are in the same decompression limit enforcement code but involve non-overlapping logic errors.
Users who followed the CVE-2020-9840 workaround by switching to .ratio(N) are affected by this vulnerability.
Patches
Fixed in swift-nio-extras 1.34.1. The fix unifies the request and response decompressor implementations so that both accumulate actual compressed bytes received (compressedLength += part.readableBytes) rather than relying on any header-supplied value.
Workarounds
Use .size(N) instead of .ratio(N) if a fixed upper bound on decompressed output is acceptable for the application. The .size limit is not affected by this vulnerability as it does not reference Content-Length.
Credits
NIOExtras is grateful to @nathanielmiller23 for their reporting and assistance with the process.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦SwiftURL | github.com/apple/swift-nio-extras | all versions | 1.34.1 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/apple/swift-nio-extras. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update github.com/apple/swift-nio-extras to 1.34.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-28975 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether CVE-2026-28975 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to CVE-2026-28975. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-28975 in your dependencies?
O3 detects CVE-2026-28975 across SwiftURL dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.